This invention relates to pressure sensitive circuit interrupters for capacitors.
Self-healing metalized film capacitors typically function by using electrical energy to burn away small capacitor faults in their metal deposited plastic film cores. This burning process tends to clear the fault, but over time the insulating material breaks down and produces gases that gradually increase the internal pressure of the sealed capacitor case. If not alleviated, the increased pressure could eventually cause the case to rupture and leak liquid insulating material. More serious capacitor faults may cause rapid breakdown of the insulating material, in turn, causing rapid build-up of internal pressure leading to a catastrophic failure of the capacitor case. Pressure sensitive interrupters (PSI) are typically incorporated in the capacitor case to prevent rupture by rapidly breaking the internal electrical connection to the capacitor element when internal pressure exceeds a predetermined limit. Thereby, further internal pressure build-up is checked by disabling the source of the pressure.
Some pressure sensitive interrupters employ restrained metal tabs connected to "button" terminals in a manner that causes the "button" terminals to separate from the tabs when an overpressure condition occurs within the capacitor case. Other interrupters employ a nicked wire inside the capacitor case which is caused to stretch and break by an overpressure condition. One end of the nicked wire typically terminates in a solder connection to an electrical terminal which protrudes through the top of the capacitor case and is used to electrically connect the capacitor to an electrical circuit. The other end of the nicked wire is held in place inside the capacitor, usually by the capacitor core. A pressure sensitive mechanism responds to overpressure by causing the electrical terminal to move, against a metered force, away from the stationary capacitor core, thereby stretching and breaking the wire. The precise length of the nicked wire needs to be controlled to assure that the expected displacement of the electrical terminal caused by overpressure provides sufficient breaking tension on the nicked wire, while preventing nuisance breakage due to small displacements caused by external mechanical pulling on the terminal.
Nicked wire interrupter systems are typically employed in metal case capacitors where the precise length of the nicked wire can be controlled by compensating for component tolerances. One compensation method uses a groove embossed around the circumference of the metal case to hold the capacitor core a precise distance from the electrical terminal located on the case lid; a retainer disk engages the groove, and prevents the capacitor core from moving toward the cover. This allows automated assembly machines to pre-cut the wires, running between the capacitor core and the terminals, to precise length before assembly. Pre-cutting the wires is important where the electrical terminals used make it difficult to access and trim the wires after assembly (e.g., three and four sided "quick connect" terminals). A pre-cut wire cut too long will interfere with operation of the terminal, while a pre-cut wire cut too short will result in a mechanically weak solder connection, possibly leading to failure of the pressure interruption mechanism.